Last lines of sonnet recalled trapping end of penis in flies (7)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

An envelope (‘trapping’) of S (the first in the answer, ‘end of peni S ‘) in TETSES, a reversal (‘recalled’) of SESTET, the second half, six lines, of an Italian ‘sonnet’, following an octave, eight lines. Shades of There’s Something about Mary – well, too close for comfort.